1. Field of the Invention
The present invention is related to a hybrid reactive power compensation device including a passive type reactive power compensator and an active type reactive power compensator serially connected thereto, which are adapted to supply a linearly adjustable reactive power within a predetermined range in the distribution power system. Moreover, the present invention is related to a hybrid reactive power compensation device including an active type reactive power compensator adapted to adjust a current flowing through the passive type reactive power compensator to be approximated as a sinusoidal waveform, and thereby it can avoid the power resonance generated between the passive type reactive power compensator and the reactance of power system that may cause destruction of the reactive power compensation device itself and adjacent power facilities.
2. Description of the Related Art
Most loads in a distribution power system have the characteristic of inductance, and it will result in a poor power factor. Hence, it requires a larger current for the identical real power that reduces the power efficiency of the distribution power system and degrades the performance of voltage regulation of the load side. For solving the above problems, power substations and power consumers generally install a passive type reactive power compensator (AC power capacitors) parallel connected to the distribution power system, so as to compensate a lagging reactive power to increase the entire power factor. In some distribution power systems, the capacity of applied AC power capacitors is about 25% to 35% of total capacity, and in some other distribution power system even exceeds about 50%, according to research reports.
Recently, harmonic pollution of industrial power system has increased seriously due to the wide use of nonlinear loads. The AC power capacitor for power factor correction provides a low impedance path for harmonic current, hence, the AC power capacitor is frequently damaged by harmonics. Meanwhile, it results in power resonance between the AC power capacitor and the distribution power system. A further result is the amplification of harmonic current and harmonic voltage. Thus, the destruction of the AC power capacitor due to over-voltage or over-current may occur. Besides, the over-voltage of AC power capacitor caused by the power resonance may destroy neighboring electric power facilities and even result in public accidents.
In order to solve the power resonance problem caused by the AC power capacitor, the voltage rating is increased to avoid the destruction of over-voltage in conventional solution. However, it cannot resolve the resonance problem and may, therefore, cause the destruction of neighboring power facilities.
There is another solution wherein the AC power capacitor is switched off from the power system when over-voltage or over-current occurs, but the function of reactive power compensation will be failed.
The reactive power compensation also can be obtained by using a set of constant AC power capacitors merely providing a fixed reactive power. This fixed reactive power cannot be adjusted to respond to the variation of loads, and it may result in over-voltage due to a light load. In order to properly adjust reactive power provided by the AC power capacitor, an automatic power factor regulator (APFR) is developed, as shown in FIG. 1. The APFR consists of a set of AC power capacitors C, through CN via switches S, through SN. Thereby the reactive power supplied from the APFR can be adjusted by changing number of AC power capacitors switched on. Although APFR can supply an adjustable reactive power to respond to the variation of loads, the APFR can merely be adjusted step by step not linearly. Therefore, the power factor of the distribution power system compensated by APFR still cannot be close unity.
Referring to FIG. 2, another power factor regulator uses a fixed capacitor parallel connected to a controllable reactor 11, which is controlled by a thyristor switch 10. This power factor regulator, so-called a Fixed Capacitor Thyristor-Controlled Reactor (FC-TCR), uses a phase control technique to control the thyristor switch 10, whereby it can provide a linearly adjustable reactive power. However, it generates a significant amount of harmonic current and results in serious harmonic pollution due to the use of the phase control technique in the thyristor.
The reactive power is adjustable in the two reactive power compensation devices described in preceding paragraphs, but the AC power capacitor thereof is parallel connected to a power system and it still cannot avoid the problem of destruction caused by the power resonance.
Referring to FIG. 3, a facility based on power electronic technology to be applied in a distribution power system to compensate reactive power, so-called the active type reactive power compensator 2, is shown. This active type reactive power compensator uses a power converter 20 via an inductor 21 to be connected to a power system 1. The power converter 20 is connected to a DC power capacitor 22 at its DC side. The active type reactive power compensator 2 may provide a leading reactive power or a lagging reactive power. The supplied reactive power can be adjusted linearly to respond to the variation of loads so that the input power factor can be maintained to be close to unity. Meanwhile, the active power factor correction system will not result in power resonance. Hence, it can avoid the destruction of the power resonance generated by an AC power capacitor. However, the active type reactive power compensator 2 must compensate the reactive power required by the loads, thus it requires a large capacity of power converter in the active type reactive power compensator. Hence, the wide application is limited due to the high cost.
The present invention intends to provide a hybrid reactive power compensation device used for supplying linearly adjustable reactive power within a predetermined range. Meanwhile, the hybrid reactive power compensation device includes an active type reactive power compensator to adjust a current flowing through a passive type reactive power compensator to be approximated as a sinusoidal waveform, and thereby it can avoid the power resonance generated between the hybrid reactive power compensation device and the reactance of power system. Therefore, it can avoid the destruction of hybrid reactive power compensation device itself and the neighboring power facilities by the power resonance. Moreover, the manufacture cost of the present invention is less expensive than that of the conventional active type reactive power compensator.